An electric motor typically includes a stator and a rotor rotatable with respect to the stator. The stator has multiple magnetic poles that are typically formed by permanent magnets. The rotor includes a shaft with a rotor core and a commutator fixed thereon. The rotor core has multiple teeth and rotor windings wound about the teeth. The windings are electrically connected to the commutator. Brushes are provided to slidably contact the commutator to feed electricity to the windings, and therefore power the motor.
A typical rotor core is formed by stacking a plurality of laminations that have an annular portion with a central hole into which the shaft is pressed. The teeth extend from a radially outer circumference of the annular portion. Motors having this type of rotor core have a disadvantage that torque ripple generated at the teeth due to the interaction with the stator is transferred to the shaft and then to the driven device, such as a fan or the like. As a result, undesirable noise is generated by the torque ripple.
Therefore, there is a desire for an improved motor that can overcome the above described shortcomings or at least provide a useful choice.